Cooling is an important means for ensuring that a high-density integrated electronic device such as a CPU is kept within its operational temperature limit. Generally, the types of apparatuses for cooling is selected from extruded metal heat sinks, fans, water, air conditioning, heat pipes, etc. Compared to heat pipes, the other cooling units have certain inherent limitations such as large size, heavy weight, and low heat transferring efficiency. A typical heat pipe generally includes a wick structure attached an inside wall of a shell, and a working fluid received in the wick structure.
In the heat pipe, the working fluid is utilized as a heating medium to transfer heat by an evaporation-condensation cycle with the help of capillarity action. The types of working fluid generally are a main factor in determining the operation performance of the heat pipe. Nowadays, a variety of working fluid may be suitable used, depending on the particular application. The basic requirements for working fluids are as follows:                Compatibility with the wick and the wall of the shell materials;        Good thermal stability;        Wettability of the wick and the wall;        High latent heat;        High thermal conductivity;        Low liquid and vapor viscosities;        High surface tension.        
The working fluids employed in the heat pipe typically include water, ammonia, methanol, acetone, heptane, etc. However, none of the working fluids is capable of satisfying all of the aforementioned requirements. This limits the heat-transferring efficiency of heat pipe. Consequently, some other techniques have been developed to overcome the limitation. For example, China Patent Application No. 02137681.6 discloses a nano-fluids having nanoparticles incorporated therein. The nano-fluid employs water or ammonia as the working fluid. Nanoparticles are dispersed in the working fluid. The nanoparticles are made of a material selected from the group consisting of silicon carbide, alumina, and magnesium oxide. A thermal conductivity of the overall working fluid may be increased to a certain extent due to the nanoparticles. However, the nanoparticles effectively make no contribution for improving the viscosity and diffusivity of the working fluid. In other words, the nanoparticles are incapable of improving the fluidity of the nano-fluids. In addition, the nanoparticles are unable to fully exhibit their excellent properties due to the limitation of the conventional working fluid.
In view of properties limitations of the conventional working fluid has led people to seek other suitable thermally conductive materials for use as working fluid in a heat pipe. For example, U.S. Pat. No. 6,530,420 issued on Mar. 11, 2003 discloses a heat carrier including a closed or looped heat pipe having a heat receiving section and a heat radiating section, and a heating medium received in the heat pipe. The heating medium becomes a low-viscous supercritical fluid when it is heated. The closed or looped heat pipe is entirely covered with a thermal insulation layer, except for the heat receiving section and the heat radiating section. Carbon dioxide is used as the heating medium received in the looped heat pipe. Carbon dioxide in the looped heat pipe becomes a very convective low viscous supercritical fluid under a pressure above 7.3 MPa and at a temperature of about 31° C. When a temperature gradient is created in the supercritical fluid in the looped heat pipe, it creates a density gradient. This causes the fluid to undergo natural convection without any external driving means, thereby transporting heat from the heat receiving section to the heat radiating section while circulating through the looped heat pipe. The supercritical fluid has a low viscosity, which is capable of enhancing the heat-transferring efficiency. However, the thermal conductivity of the supercritical fluid is relatively low. This limits the overall heat-transferring efficiency of the heat pipe.
What is needed, therefore, is a thermally conductive material which has both excellent thermal conductivity and fluidity.